In the TA, A3 has a select plane set availible for flight from a 30K take off. There is also a 20k airspawn in the bomber zone. That's a good place to get a feel for planes at altitude without the climb time.
Something I find important for high alt fighting is keeping track of BOTH airspeeds much more closely than at low altitudes. IAS gives you an idea of your maneuver options. For instance, no matter how fast your true airspeed is, if IAS says you're doing 160mph, then an immelman is probably not an option. On the flip side if you're considering a maneuver that will increase your speed, you want to look at TAS. If you're already doing 420mph, it might not be a good idea to pull a split-s. Depending on your airframe, you may end up losing several thousand feet until you can recover level flight.